Two-and-a-half billion years ago, the Earth’s atmosphere was rich in hydrocarbons, similar to Saturn’s moon, Titan. Before Earth’s atmosphere ditched methane and began accumulating oxygen, though, our planet appears to have cycled back and forth every few million years between the two states years a hydrocarbon haze and clear skies. A sunlight-blocking haze most certainly affected the evolution of microbes that depend on light to photosynthesise and contributed to the delay before the final oxygenation of the atmosphere.

But some 2.4 billion years ago, photosynthesising microbes generated enough oxygen for it create the “great oxygenation event.” But the reason for that lag, says bio-geochemist Aubrey Zerkle of Newcastle University, UK, “is one of the great mysteries of Earth’s history.”

Zerkle and colleagues have analyzed the ancient atmospheric conditions before that oxygenation event, by analysing the chemical make-up of a core of ocean sediment deposited on a region of South Africa which was flooded between 2.65 and 2.5 billion years ago, when microbes ruled a methane-bound world and Earth’s continents began forming in earnest.

During the 150-million-year interval, the researchers found that the oxygen produced by photosynthetic organisms was staying in the ground rather than entering the atmosphere when methane dominated the atmosphere, resulting in the thick hydrocarbon fog. But at other times during the interval, the amount of methane in the atmosphere decreased and the haze cleared.